1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas for removing nitrogen oxides, carbon monoxide and hydrocarbons contained in the exhaust gas discharged from an internal combustion engine such as an automobile engine. More specifically, it relates to a catalyst for purifying an oxygen-rich combustion exhaust gas.
2. Description of the Related Art
Nitrogen oxides, carbon monoxide and hydrocarbons, as detrimental materials in an exhaust gas discharged from an internal combustion engine, are conventionally removed by a ternary catalyst comprising Pt, Rh, Pd, and the like, supported on a support. Nevertheless, effective catalysts for the removal of nitrogen oxides in the exhaust gas of a Diesel engine were not available, because the exhaust gas contains large quantities of oxygen, and accordingly, an exhaust gas purification using a catalyst was not possible.
A lean fuel combustion (i.e., combustion at a relatively low fuel/oxygen ratio) has become necessary for recent gasoline engines, to obtain a low fuel consumption and reduce the quantity of the carbonic acid gas emitted, but since the exhaust gas of this lean combustion gasoline engine is oxygen-rich, the conventional ternary catalysts as described above cannot be employed, and thus a practical method of removing these detrimental components has not yet been discovered.
As a method of removing, in particular, nitrogen oxides in such an oxygen-rich exhaust gas, there are known a method which adds a reducing agent such as ammonia and a method which adsorbs and removes the nitrogen oxides by using an alkali, and so forth, but these methods are not suitable for use in an automobile, which is a moving source of the generation of an exhaust gas, and the application thereof is limited.
It is known in the art that a zeolite catalyst prepared by an ion-exchange with a transition metal can be used in the same way as the conventional ternary catalyst. Japanese Unexamined Patent Publication (Kokai) No. 1-130735, for example, proposes a catalyst which can selectively reduce the nitrogen oxides even in an oxygen-rich exhaust gas containing a trace amount of a reducing agent such as unburnt carbon monoxide and hydrocarbons.
Nevertheless, the catalyst proposed in this prior art reference undergoes a remarkable lowering of its activity in the course of long time use at a high temperature, and needs much improvement from the aspects of durability and catalyst performance. Accordingly, this catalyst has not been put to practical use.